Pella Door won’t lock (3 point lock)

I'm going to share this question and answer it myself because someone else may have had this problem.

I have several Pella doors in my home with the 3 point locks (top and bottom "shootbolts" and a dead bolt near the handle). They have been problematic. The locking mechanism on a couple of rarely used doors would hardly work anymore (very stiff and wouldn't lock anymore). So I disassembled them, cleaned up the runners and other parts, applied a good lube, reassembled and they worked again. They are a pain to take apart, re-adjust and reassemble.

Recently they started doing the same thing. I though oh no, not again. I didn't initially consider there might have been a new cause. What could that cause be?

OK, that's the teaser, I'll answer my own question straight away (as our British friends would say). I hope that's the right approach for SE, I think I saw somewhere you could answer your own questions.